Rambam (Sefer HaMadda, Hilkhot Yesodei HaTorah 1:4)
states: ‘“For the Lord God is true’ – He alone is the truth, and nothing
else possesses truth like His truth.
This is what is meant when the Torah states, ‘There is no other besides Him’ (Deut. 4:3s).
It means that there is no other entity besides Him that is true as He is.”
The phrase “The Lord God is true” implies that God is the only truth, relative to which
all else is not true.
The phrase “For the Lord God is true” also indicates that God is the true and exclusive existence, as there is absolutely no other existence outside of Him.
Rabbi Adin Steinsaltz